[time-nuts] Re: 100 MHz Low Phase Noise Mobile GPSDO/GNSSDO?

[Carsten Andrich]

Hi Markus, Jim,

thank you for your responses!

On 19.04.22 11:53, Markus Kleinhenz via time-nuts wrote:
> I believe you will have a hard time finding a COTS solution that fits
> most of your requirements.
>
> You will likely have to split your signal chain into multiple devices:
>
> 1. UBlox ZED-F9T based GPS-Receiver with 1PPS out
> 2. Something like a JacksonLabs RCM (a ULN Oscillator that locks to a
> externally provided 1PPS)
> 3. Distribution Unit
>
> So you can spread your requirements.
Thanks for the suggestion. I've had a quick look at the RCM. 
Unfortunately, its "1PPS synchronization counter resolution" is ±2.5ns, 
which is below what a ZED-F9T can do with pulse quantization correction 
(~4e-10 ADEV @ 1s) [1]. I presume a short PLL time constant and 
therefore high resolution are key here to keep the OCXO within 1 ns of 
the GNSS despite environmental disturbances (temperature, acceleration, 
vibration). I think I'll have to realize the 1PPS tracking and OCXO 
disciplining myself with a sufficiently accurate time interval counter 
(e.g., TI TDC7200, which has ~50 ps resolution).
> Another big challenge will be the 1ns relative accuracy between multiple
> (moving!) devices. Without communication between devices this should not
> be possible.
We've conducted RF measurements with a bunch of stationary(!), but 
spatially distributed (1~2 km) SRS FS740 GNSSDOs (with Rb option) back 
in 2019 and have achieved better than 5 ns synchronization accuracy 
(verified using line-of-sight RF measurements) without communication 
between the FS70 [2], simply because the FS740 does not support that. Of 
course that's not really comparable to 1 ns with moving GNSSDOs, but is 
already pretty decent for the FS740s' single-band GNSS receiver with 15 
ns 1PPS timing accuracy (likely Trimble RES SMT 360).

On 19.04.22 15:49, Lux, Jim wrote:
> I think that 1 ns might be achievable, if the receivers are in the 
> same general area, so they see the same propagation, but perhaps not 
> as an "off the shelf" device.
That's where the ZED-F9T's dual-band reception and differential timing 
feature come into play, which improve the time pulse accuracy to 2.5 ns. 
It's similar to real-time kinematic positioning (DGNSS on steroids) by 
distributing a correction signal to multiple receivers.
> We are getting <1ns with post processing using GIPSYx in a weak signal 
> environment with generally poor geometry from GNSS standpoint (we're 
> going to be in GEO, above the constellation, so the signals we see are 
> on the other side of Earth).

Unfortunately, post-processing is not really an option for me, as long 
as I can avoid it. I think the ZED-F9T is as good as it gets in terms of 
value-for-money. I'll keep you updated when I make any progress on 
wiring up a proof-of-concept.

Best regards,
Carsten

[1] 
https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf#page=25
[2] https://ieeexplore.ieee.org/document/9128562 <--- Behind IEEE 
"paywall"; I can send you a personal copy of the paper upon request